1. Field of the Invention
The present invention relates to electronic circuits and systems. More specifically, the present invention relates to radio frequency amplifiers used in communications systems.
2. Description of the Related Art
Signal integrity is a critical consideration in the design and operation of amplifier circuits, particularly when used in transmitters and receivers for communications systems. For a variety of applications, it is imperative that the output of the amplifier accurately tracks the input. Inevitably, noise and distortion introduce inaccuracies in the output signal, which will then be propagated throughout the rest of the system. In this connection, the term xe2x80x98noisexe2x80x99 refers to the thermal noise generated by the circuit, while xe2x80x98distortionxe2x80x99 refers to the nonlinearities produced by the circuit design such as harmonics or intermodulation products.
Currently, conventional low noise amplifiers are designed to keep the thermal noise level to a minimum by using larger transistors. The large signal currents in the larger transistors, however, inherently produce high intermodulation distortion, particularly at high (e.g., radio) frequencies. There is currently no method for simultaneously minimizing both noise and distortion in an RF amplifier.
Hence, a need remains in the art for an improved radio frequency amplifier design offering wider bandwidth, lower noise, and lower distortion.
The present invention uses a bootstrap design technique in a low noise radio frequency amplifier in order to achieve a wide bandwidth, low noise figure, and low distortion all at once.
In the illustrative embodiment, the invention includes a first circuit for receiving an input signal; a second circuit for amplifying the input signal using a transistor Q2; and a third circuit for regulating a rate of change of voltage across the transistor Q2 such that the rate of voltage change is zero. The third circuit includes a transistor Q3 connected to the transistor Q2 in cascode. In the specific illustrative embodiment, the third circuit further includes two diodes D1 and D2 used to modulate the voltage at the input of the transistor Q3 in proportion to the voltage modulation at the input of the transistor Q2. Thus, the rate of voltage change across the transistor Q2 is zero, and the high intermodulation distortion normally generated in a conventional low noise amplifier is eliminated. The transistor Q2 can then be enlarged to minimize thermal noise without reducing bandwidth or degrading distortion performance.
In the illustrative embodiment, the second circuit includes a transistor Q1 connected in cascade to the transistor Q2. The transistor Q1 buffers the input from the high current running from the transistor Q2. In the specific illustrative embodiment, the invention further includes a fourth circuit for regulating a rate of change of voltage across the transistor Q1 such that the rate of voltage change is zero. The fourth circuit includes a transistor Q4 connected to the transistor Q1 in cascode. The two diodes D1 and D2 also connect the transistors Q1 and Q4 such that the voltage at the input of the transistor Q4 is modulated in proportion to the voltage modulation at the input of the transistor Q1. Thus, the rate of voltage change across the transistor Q1 is zero, and the transistor Q1 can also be enlarged to minimize thermal noise without reducing bandwidth or degrading distortion performance.
The present invention provides a larger bandwidth, a lower noise figure and lower distortion than what has been achievable up to this point in time. These advantages are afforded by the fact that a bootstrap design is used to regulate the voltage excursions across the main noise-setting, distortion determining transistor Q2. The size of Q2 can then be enlarged to reduce noise without generating distortion. In addition, the bootstrap of the transistor Q1 allows its size to also be enlarged to reduce noise without degrading distortion performance.